We will continue our phylogenetic survey of invertebrate muscle to determine the distribution, content relative to myosin, subcellular location and degree of evolutionary conservatism of the muscle protein, paramyosin. This will be accomplished using SDS polyacrylamide gel electrophoresis, light and electron microscopic immunohistochemistry and immunochemical techniques. We will attempt to relate paramyosin content to thick filament dimensions and molecular organization in a variety of muscles by studying isolated and reconstituted thick filaments by electron microscopy and optical diffraction. Further, we will study, both in situ and in vitro, factors regulating such phenomena as A-band and thick filament shortening which have been observed in paramyosin containing muscles. Based on these studies and those of our collaborators, Dr. Maynard M. Dewey and Dr. Benjamin Walcott, S.U.N.Y., Stony Brook, N.Y., we will define the relationship between the organization of elements of the contractile apparatus, their relative paramyosin content and the physiological properties of different muscle types. We hope to gain an understanding of the role of paramyosin in those mucles in which it is found.